Self-assembly of an exchange-spring composite via magnetic phase separation in Pr1-xCaxCoO3
S. El-Khatib1,2, S. Bose3, C. He3, J. Kuplic3, J. Lynn1, J. Borchers1,
J.F. Mitchell4 and C. Leighton3
1 NIST Center for Neutron Research, NIST
2Materials Science, University of Maryland
3 Materials Science, University of Minnesota
4 Materials Science Division, Argonne National Lab
We report structural and magnetic properties in bulk polycrystalline Pr1-xCaxCoO3 (0.00 £ x £ 0.30) from x-ray and neutron diffraction, thermogravimetic analysis, magnetometry, and small-angle neutron scattering (SANS). Upon cooling, the Pr0.70Ca0.30CoO3 composition (deep in the ferromagnetic (FM) phase) first undergoes a transition around 250 K where short-range FM clusters emerge with a size of order 1-2 unit cells. The magnetization and SANS intensity slowly increase on cooling to 70 K, where the system undergoes a transition to a long-range ordered FM state, but with low magnetization, indicative of a small FM volume fraction. Magnetometry and SANS data indicate coexistence of the short-range clusters within a network of long-range FM. The coercivities of the short-range and long-range FM regions are very different, and a non-monotonic T dependence of the coercivity reveals clear evidence of FM exchange coupling between the phase-separated regions. In essence the phase separation in this compound leads to natural formation of a hard/soft composite, which displays classic exchange spring behavior. Work at UMN supported by DoE.